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Regular Approximation (regular + approximation)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Computational NMR Spectroscopy of Transition-Metal/Nitroimidazole Complexes: Theoretical Investigation of Potential Radiosensitizers

HELVETICA CHIMICA ACTA, Issue 10 2005
Teodorico
The computed chemical shifts of transition-metal complexes with dimetridazole (=,1,2-dimethyl-5-nitro-1H -imidazole; 1), a prototypical nitro-imidazole-based radiosensitizer, are reported at the GIAO-BP86 and -B3LYP levels for BP86/ECP1-optimized geometries. These complexes comprise [MCl2(1)2] (M,=,Zn, Pd, Pt), [RuCl2(DMSO)2(1)2], and [Rh2(O2CMe)4(1)2]. Available ,(1H) and ,(15N) values, and ,,(1H) and ,,(15N) coordination shifts are well-reproduced theoretically, provided solvation and relativistic effects are taken into account by means of a polarizable continuum model and suitable methods including spin,orbit (SO) coupling, respectively. These effects are particularly important for the metal-coordinated N-atom, where the contributions from solvation and relativity can affect ,(15N) and ,,(15N) values up to 10,20,ppm. The 195Pt chemical shifts of cis - and trans -[PtCl2(1)2] are well-reproduced using the zero-order regular approximation including SO coupling (ZORA-SO). Predictions are reported for 99Ru and 103Rh chemical shifts, which suggest that these metal centers could be used as additional, sensitive NMR probes in their complexes with nitro-imidazoles. [source]

Note on the calculation of analytical Hessians in the zeroth-order regular approximation (ZORA)

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 12 2006
J. H. van Lenthe
Abstract The previously proposed atomic zeroth-order regular approximation (ZORA) approach, which was shown to eliminate the gauge dependent effect on gradients and to be remarkably accurate for geometry optimizations, is tested for the calculation of analytical second derivatives. It is shown that the resulting analytic second derivatives are indeed exact within this approximation. The method proves to yield frequencies that are remarkably close to the experimental frequency for uranium hexafluoride but less satisfactory for the gold dimer. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]

Relativistic effects in the optical response of HgSe by time-dependent density functionals theory

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001
P. L. de Boeij
Abstract We treat the dominant relativistic effects in the optical response properties of mercury selenide using time-dependent density functional theory (TDDFT). The scalar relativistic effects have been included within the zeroth-order regular approximation (ZORA) in both the ground-state DFT calculations and in the time-dependent response calculations. Within this approximation the HgSe crystal is found to be a semimetal. In a previous study [J Chem Phys 2001, 114, 1860] we have shown that TDDFT/ZORA can be applied successfully to narrow-gap semiconductors, such as indium antimonide, that become semimetallic within the local density approximation when scalar relativistic effects are included. Results are given for the band structure, the static dielectric constant ,,, and the dielectric function ,(,) of HgSe, and these results are compared with the similar ones for InSb. We find considerably improved results for the dielectric function of HgSe when relativistic effects are included. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source]

103Rh NMR Chemical Shifts in Organometallic Complexes: A Combined Experimental and Density Functional Study

CHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2004
Laura Orian Dr.
Abstract Experimental 103Rh NMR chemical shifts of mono- and binuclear rhodium(I) complexes containing s - or as -hydroindacenide and indacenediide bridging ligands with different ancillary ligands (1,5-cyclooctadiene, ethylene, carbonyl) are presented. A protocol, based on density functional theory calculations, was established to determine 103Rh NMR shielding constants in order to rationalise the effects of electronic and structural variations on the spectroscopic signal, and to gain insight into the efficiency of this computational method when applied to organometallic systems. Scalar and spin,orbit relativistic effects based on the ZORA (zeroth order regular approximation) level have been taken into account and discussed. A good agreement was found for model compounds over a wide range of chemical shifts of rhodium (,10,000 ppm). This allowed us to discuss the experimental and calculated ,(103Rh) in larger complexes and to relate it to their electronic structure. [source]

Inside a Superatom: The M7q (M=Cu, Ag, q=1+, 0, 1,) Case

CHEMPHYSCHEM, Issue 3 2010
Alvaro Muñoz-Castro Dr.
Abstract All-electron relativistic density functional calculations are performed to obtain the electronic structure and nucleus-independent chemical shifts (NICS) of D5hpentagonal-bipyramidal (PBP) Cu7qand Ag7q(q=1+,0,1,) clusters. Scalar and spin,orbit relativistic effects are taken into account at two levels: the two-component zero-order regular approximation (ZORA) Hamiltonian and fully relativistic four-component calculations via the Dirac equation. These clusters are treated by including the spin,orbit effect in the jellium model, within the double-valued point group (D5h*) establishing the symmetry correlations between the molecular and the atomic spinors given by the full rotation group. These clusters show highly spherical aromaticity, which is suggested to increase the hardness of the superatom. Thus, the calculations suggest that the paramagnetic Cu7 and Ag7 clusters can be regarded as pseudohalogens. [source]